OXON HILL, Md.— A newly discovered exoplanet weighs about the same as Earth — but that’s where the similarities end. Its discovery, reported January 6 at the American Astronomical Society meeting and online at arXiv.org, illustrates the complexities of classifying planets outside the solar system.

The planet, KOI-314c, orbits a small, cool star 200 light-years away in the constellation Lyra. While the planet’s mass is similar to Earth’s, its diameter is 60 percent larger, said David Kipping, a...